<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<style>
	#div1 {width: 100px; height: 150px; background: red; position: absolute;right: 0; bottom: 0;}
</style>
<script>
	window.onscroll = function()
	{
		 var oDiv = document.getElementById('div1');
			var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
			//oDiv.style.top = document.documentElement.clientHeight - oDiv.offsetHeight + scrollTop + 'px';
		 startMove(document.documentElement.clientHeight - oDiv.offsetHeight + scrollTop);
	};
	
	var timer = null;
	function startMove(iTarget)
	{
		 var oDiv = document.getElementById('div1');
			clearInterval(timer);
		 timer = setInterval(function(){
					var speed = (iTarget-oDiv.offsetTop)/7;
					speed =speed>0?Math.ceil(speed):Math.floor(speed);
					
					document.title =oDiv.offsetTop + ',' + speed;
					if(speed==0)
						{
							clearInterval(timer);
						}
				else
					{
						//虽然可以写在if外面， 但是可以少执行一次代码。 因为是缓冲运动， speed 肯定会到0,speed等于0  意味着移动到了指定位置
						oDiv.style.top=oDiv.offsetTop + speed + 'px';
					}
			},30);
	}
</script>
</head>

<body style="height: 2000px">
<div id="div1"></div>
</body>
</html>
